A photosensitive resin composition is used as a material for forming electronic components, optical products or optical elements, a material for forming layers, an adhesive, etc. Especially, it is suitably used as a material for products or components that are formed through a patterning process using electromagnetic radiation.
For example, a polyimide is a polymer material and its properties such as heat resistance, dimensional stability and insulation property are top-ranking in organic materials. Thus, it is widely used as an insulation material for electronic components, etc., and it is increasingly and actively used as a chip coating film of semiconductor elements, a substrate of flexible printed-wiring boards, and so on.
Also in recent years, to solve a problem with polyimide, intense investigations have been carried out into polybenzoxazole having a low water absorption property and a low dielectric constant, polybenzimidazole having excellent adhesion to a substrate, and the like, which can be processed in a similar manner to polyimide.
In general, polyimide has poor solubility in solvents and is difficult to process. As the method for patterning polyimide into a desired shape, therefore, there is a method for obtaining a pattern of polyimide by performing patterning by exposure to light and development when it is in the form of polyimide precursor that has excellent solubility in solvents, and then imidizing the same by heating, etc.
Various methods are proposed for forming a pattern by using a polyimide precursor. Two typical examples thereof are as follows:
(1) A pattern forming method in which, when a polyimide precursor has no ability to form a pattern, a photosensitive resin is provided on a polyimide precursor in the form of resist layer.
(2) A pattern forming method in which a photosensitive site is bonded to or coordinated with a polyimide precursor and a pattern is formed by its action. Alternatively, a pattern forming method in which a polyimide precursor is mixed with a photosensitive component to produce a resin composition, and a pattern is formed by the action of the photosensitive component.
Typical patterning methods using (2) include: (i) a method in which a naphthoquinonediazide derivative that acts as a dissolution inhibitor before exposure to electromagnetic radiation and forms a carboxylic acid to act as a dissolution promoter after the exposure, is mixed with a polyamic acid (polyimide precursor) so that the contrast between the dissolution rate of an exposed region in developers and that of an unexposed region is increased, thereby forming a pattern; thereafter, the pattern is imidized to obtain a polyimide pattern (patent literature 1) and (ii) a method in which a methacryloyl group is introduced into a polyimide precursor via an ester bond or ionic bond and a photoradical generator is added thereto to crosslink exposed regions so that the contrast between the dissolution rate of the exposed region in developers and that of an unexposed region is increased, thereby forming a pattern; thereafter, the pattern is imidized to obtain a polyimide pattern (patent literature 2).
Compared with method (1), method (2) needs no resist layer, so that the process can be significantly simplified. However, method (i) is problematic in that polyimide's characteristic properties are not obtained when the added amount of the naphthoquinonediazide derivative is increased for increasing the dissolution contrast. Method (ii) is problematic in that there is a limitation on the structure of the polyimide precursor.
There is a report of other patterning method (iii) in which a polyamic acid (polyimide precursor) is mixed with a photobase generator, exposed to light and then heated to promote cyclization by the action of bases generated by the exposure and thus to decrease the solubility of the polyimide precursor in developers, so that the contrast between the dissolution rate of the exposed region in developers and that of the unexposed region is increased, thereby forming a pattern; and then the pattern is imidized to obtain a polyimide pattern (patent literature 3).
Other examples of the photosensitive resin composition comprising a photobase generator include a photosensitive resin composition comprising an epoxy compound (for example, patent literature 4). A photobase generator is exposed to light to generate amines in a layer that contains an epoxy compound, so that the amines act as an initiator or catalyst and cure the epoxy compound in the exposed region only, thereby forming a pattern.    Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. S52-13315    Patent Literature 2: JP-A No. S54-145794    Patent Literature 3: JP-A No. H08-227154    Patent Literature 4: JP-A No. 2003-212856